Implicit dataset from environ

[dhermes]

This has #436 as a diff-base, so feel free to hold off review until it is merged.
This adds helper methods for implicit datastore environ.

By setting two environment variables

• GCLOUD_DATASET_ID
• GOOGLE_APPLICATION_CREDENTIALS

the user can call convenience methods in gcloud.datastore directly
without worrying about auth or the name of the dataset. The
goal is that in places like App Engine and Compute Engine, the
dataset ID can be implied without any user intervention.

Partially addresses #337.

NOTE: This still needs to be documented, but it's unclear where is appropriate. We also need to have documentation for auth (outside of CONTRIBUTING.md).

[coveralls]

Coverage remained the same when pulling 2f227ac on dhermes:implicit-dataset-from-environ into 89ded75 on GoogleCloudPlatform:master.

[dhermes]

Also note this can't be merged until the comment in Entity.__init__ is resolved:

    def __init__(self, dataset=None, kind=None, exclude_from_indexes=()):
        super(Entity, self).__init__()
        # DJH: Need to decide if this inherits from object/dict. Notice that
        #      `Entity` objects are False-y even if they have protected
        #      variables set.

This is the question put forth in #396. The short term solution is to inherit from both dict and the helper that Query and Transaction inherit from:

class Entity(dict, _implicit_environ._DatastoreBase):
    ...

Though branching MRO is something to avoid if we can.

[coveralls]

Coverage remained the same when pulling 13c727b on dhermes:implicit-dataset-from-environ into 21e5f45 on GoogleCloudPlatform:master.

[dhermes]

@silvolu This was one of the "convenience" things I mentioned before.

[coveralls]

Changes Unknown when pulling f0a7825 on dhermes:implicit-dataset-from-environ into * on GoogleCloudPlatform:master*.

[dhermes]

@tseaver PTAL

[coveralls]

Coverage remained the same when pulling 153f306 on dhermes:implicit-dataset-from-environ into b343acf on GoogleCloudPlatform:master.

[coveralls]

Coverage remained the same when pulling c5457aa on dhermes:implicit-dataset-from-environ into b343acf on GoogleCloudPlatform:master.

[coveralls]

Coverage remained the same when pulling d85d200 on dhermes:implicit-dataset-from-environ into b343acf on GoogleCloudPlatform:master.

[tseaver]

I'm pretty opposed to "implicit majyk" that triggers at import time, with no way for the application to control or opt out of. I'd much rather have the application do an explicit configuration call, either:

from gcloud.datastore import guess_default_dataset
guess_default_dataset(os.environ)

or have a way to register the dataset ID explicity (e.g., from a value in a config file)::

from gcloud.datastore import set_default_dataset
set_default_dataset(config.settings['gcloud.dataset_id'])

Yes, it requires a tiny bit more typing on a host where the environment contains the "right stuff", but it makes the process explicit, and a lot easier to explain / understand.

[coveralls]

Coverage remained the same when pulling 7cd77f2 on dhermes:implicit-dataset-from-environ into b343acf on GoogleCloudPlatform:master.

[dhermes]

@tseaver I agree explicit is better, PTAL at latest changes. Thanks for the insight, I could have put more effort into the design of the interface.

RE: The MRO comment in Entity.__init__ it's moot since we are dropping dataset from Entity long term.

---

Also note RE: defaults set by free functions in datastore.__init__: when we remove Dataset we will need to also provide a method which sets a default connection and then have things like

q = datastore.query('Kind')

which will use both the default dataset ID and the default connection (bound to the query).

It's more likely that it'll only be necessary to use the default Connection in

q.fetch()

and just provide a

q.fetch(connection=connection)

to override.

[tseaver]

LGTM, with the note on raising an exception in 'set_default_dataset' if no value is passed and we can't figure one out from the environment.

[dhermes]

Going to merge this, then rebase in #456 and merge that.

I'm not 100% sure about raising when set_default_dataset fails. You're probably right, but that's an easy change and GCE and GAE support still need to be added to the method so there is a lot to do.